1. Field of the Invention
The present invention relates to electromechanical etching, where an image is read by reflection of radiation and resulting electronic signals are used to engrave an image on a metal plate or drum.
2. Background Art
One method for producing gravure printing plates consists of an electromechanical process in which there are a series of steps before the plate is made. An original image is converted to a positive or negative on a highly reflective sheet, called opalescent film. The imaged opalescent film is then placed on a scanning drum (usually having a highly reflective white surface) and point-by-point or area-by-area reflective optical densities are read by a densitometer. This information is immediately transmitted or stored in electronic form. The information is eventually transmitted to a mechanical engraver. The electronic signals or impulses tell the engraving apparatus what to do. One commercially available form of this apparatus, the Helio-Klischograph, uses a diamond pointed stylus that vibrates at between 3000 and 5000 times per second. It is therefore capable of cutting up to 5,000 tiny inkwells on the surface of a metal plate or cylinder each second. The inkwells or pits vary in size and depth according to the specific electronic signals originating from the electronic eye within the scanner. The final gravure plate is formed by this etching of the surface of the plate to produce an image corresponding to the read image of the substrate. With this particular type of equipment, the pit is a characteristic pyramidal shape when viewed under a microscope, as compared to the saucer or oval shaped pits produced by chemcial etching.
The need to make the intermediate introduces the risk of additional errors. The required exposure to a light source and chemical treatment (development) of the photosensitive material introduce variable that can affect the quality of the intermediate. The direct use of the transparent screened film would eliminate those variables and be more convenient.
It has been reported by the Gravure Research Institute (Report No. M-243 "Opaque Photographic Products for Electromechanical Engraving: Continuous Tone and Halftone", Nov. 9-12, 1983, 36th Annual Meeting) that most printers experienced considerable difficulty when attempting to use transparency halftone film as the film to be scanned while engraving the metal plate. The use of a halftone original would enable the process to be performed without having to produce the highly reflective image from the original. The source of these problems were reported to be related to varying reflective indices due to (1) different types of film being scanned simultaneously, (2) different chemistries being used, and (3) a lack of transparency of some of the films used. In particular, multiple reflections off protective topcoats, antihalation layers and subbing layers was indicated as generating spurious reflective images which accompanied the reflected scanning image of the halftone image.